The present invention relates to improvements of a workpiece feeding device in a sewing machine installed with a lower and an upper feeding mechanism, and more particularly to a feeding device comprising a driven upper endless feeding member such as a wheel or a tape steadily contacting the upper surface of a workpiece.
In general such endless feeding members are either continuously driven by an interposed reduction gear or in an intermittent manner by the application of ratchet means known as one-way couplings or overrunning elements. An endless feeding member is frequently applied in combination with another feeding system, at which a feed dog adjustably performs a four-motion feeding movement. In such a combination it has found advantageous and economical to combine a four-motion feeding system with an intermittent endless feeding system, since in a simple design a one-way coupling driving the endless feeding member may be drivingly connected to the adjustable four-motion feed system. Thus, both intermittent feeding systems are drivingly synchronized and adjustably connected and, in contrary to gear driven feeding systems, a stepless feed rate adjustment is obtained.
In the U.S. Pat. No. 3,141,428 it is described a sewing machine installed with a lower and an upper wheel feed, both conveniently driven by a mechanism including one-way coupling means, by which in this particular arrangement a quasi continuous feed of a workpiece is achieved.
From the U.S. Pat. No. 3,485,193 it is known a sewing machine having a combined lower and upper feeding system, at which a lower intermittently acting feed dog cooperating with a presser foot is assisted by an upper feed roller. As illustrated and described, the upper feed roller is intermittently driven by a ratchet means connected to the lower feeding mechanism.
In a pending patent application Ser. No. 743,061, filed Feb. 7, 1977, now abandoned, there is disclosed a sewing machine installed with a four-motion feed dog and an upper endless feeding belt arranged behind or laterally to a presser foot.
All aforesaid sewing machines are installed with workpiece feeding systems comprising one-way coupling means. Besides the mentioned advantages, one-way couplings have the shortcoming to convert an oscillating input movement into an intermittently rotating movement in one direction only. Due to this fact, endless feeding members driven in such a manner must be rendered inoperative in a reversed feed mode, which becomes necessary for example on tacking operations. For this purpose, a reverse feed regulator for reversing the lower feed dog is shiftably connected with the endless feeding mechanism for simultaneously lifting the endless feeding roller or belt from the workpiece, as described in detail in the last two cited references. In order to assure a stitch formation while reverse stitching, the presser foot must remain in its lowered position. On the other hand in forward sewing mode the presser foot frictionally wipes on the upper surface of the workpiece and in conjunction with the upper feed member a stress is induced to the material and to the threads of the stitch which, especially on light materials, causes puckering. Furthermore, the arrangement of a presser foot and a feed roller or a feed belt results in an enlarged area contacting the workpiece which resistively effects the sewing of profiled stitchlines. Finally, such arrangement requires some space which is obstructious on certain operations.